Fix a deadlock with redo-oob.

If a checksummed target A used to exist but is now missing, and we tried to
redo-ifchange that exact file, we would unnecessarily run 'redo-oob A A';
that is, we have to build A in order to determine if A needs to be built.

The sub-targets of redo-oob aren't run with REDO_UNLOCKED, so this would
deadlock instantly.

Add an assertion to redo-oob to ensure we never try to redo-ifchange the
primary target (thus converting the deadlock into an exception).  And skip
doing redo-oob when the target is already the same as the thing we have to
check.

state.py

@@ -262,6 +262,7 @@ class File(object):
         reldep = relpath(dep, vars.BASE)
         debug2('add-dep: %r < %s %r\n' % (self.name, mode, reldep))
         assert(src.name == reldep)
+        assert(self.id != src.id)
         _write("insert or replace into Deps "
                "    (target, mode, source) values (?,?,?)",
                [self.id, mode, src.id])